There is an excellent book called the Continent that goes through the history of Africa and slavery. The Zulu's the most powerful tribe attacked and took as slaves the men of other tribes.
When Capt. Van Dorf came around the cape of good hope and started trading with the Zulu's the Zulu's used their slaves in exchange for material goods from Europe.
Africans kept slaves and, in areas of Africa still do to this day.
Uh, no. How about affirmative action? How about 20,000 gun control laws? How about legalized sodomy? How about busing? In local politics absurd over-zoning. The endangered species act as currently interpretted.
The book you read is not very accurate. The Zulus did not become a powerful tribe until the early 19th century, when the slave tribe was well along in the process of being destroyed by the British. They were also much too far south for the Arab slavers, who had more accessible markets.
The Zulus had slaves, but not very many. They instead generally either incorporated adopted tribes into their nation or exterminated them.
In any case, they were marginal players in the slave trade. Now the Ashanti...
"the [traders] soon found out African emperors were willing to trade fellow Africans."
Someday, (like hopefully, tomorrow, or at least day after) they may also be able to ponder on the truth, of your statement;
"Africans kept slaves and, in areas of Africa still do to this day."
When, or if that day come, it might well lead to a groundswell of support, to rise up "as one", and condemn the present day murder and oppression that SOME Africans in power, subject those Africans to, whom are unfortunate enough, to be under their thumb.